Fetal development of the mammalian skeleton is a complex embryological process which is poorly understood at the molecular level. Early in embryonic life the mammalian skeleton is first manifested as a blastema- like structure of condensing mesenchymal cells. In most regions of the embryo, this transient framework is subsequently transformed into a primarily cartilaginous structure, which itself is eventually replaced (for the most part) by bone. To understand the genetic basis of embryonic development of the mammalian axial skeleton we propose to alter the normal function of the two homeobox genes (Dlx5 and Dlx6) which show highly restricted expression in all fetal skeletal elements. In addition, we will identify and characterize the DNA cis-regulatory elements which direct regionally- restricted gene expression in the developing skeleton. To this end, we will rely primarily on the transgenic mouse technology that we have developed and refined over the last seven years. Accordingly, a) we will generate targeted disruptions (gene knock-outs) of Dlx5 and Dlx6; b) determine the DNA regulatory elements which control Dlx5 and Dlx6 skeleton-specific gene expression. Results of these experiments will help clarify the role of Dlx5 and Dlx6 in normal skeletal development. This is in keeping with our long range goal which is to understand the molecular mechanisms involved in directing mammalian fetal skeletal development with the belief that such knowledge will eventually be used to correct either trauma to, or genetic defects of the skeleton, particularly those resulting in basic osteoporosis related problems.